Recently, the popularity of mobile communications consumer devices (e.g., smart phones and tablet devices) has soared. These devices may typically encounter harsh acoustic noise environments due to their mobile nature (e.g., in cars, traffic, and street noise). The non-stationary nature of the acoustic noise demands that these devices utilize improved microphones. For example, multiple microphones may be used for noise cancellation and/or microphone arrays for beam steering.
There is also a growing trend towards ultra-thin form factors, and sleek and light-weight industrial designs, as evidenced by the popularity of the latest generations of smart phones and tablet devices. In addition, there is also pressure to reduce the bill-of-materials (BOM) cost for these devices by reducing their number of components and reducing the printed circuit board (PCB) space.
Digital micro electro-mechanical systems (MEMS) microphones have been found to meet many of these requirements described above. The digital MEMS microphones are replacing analog microphones in mobile handsets and tablet devices, and may replace the analog microphone in other technologies as well.
Digital microphones may also produce a one-bit pulse density modulated (PDM) digital output. A digital microphone capable of producing a one-bit PDM digital output takes individual samples (e.g., 8 or 12 bits worth of information) of a sound pressure wave received by the microphone and directly converts each of them into a PDM data stream of 1's and 0's. For example, each sample may be converted into a bitstream with a selected number of bits as determined by a selected oversampling ratio (OSR), such that a sample rate*OSR=the number of bits in a bitstream for each sample. A PDM data stream of all 0's would represent a maximum negative amplitude, a PDM data stream of all 1's would represent a maximum positive amplitude, and a PDM data stream of alternating 1's and 0's would represent a zero amplitude.
Smartphone and tablet platforms may also utilize more than one digital microphone. Voice communication may be signal channel and can be carried out with a single microphone voice capture in an ambient noise-free environment. However, increasingly, the consumer usage of these mobile devices is carried out in noisy environments. In order to maintain a high-quality of conversation, typical a microphone array of two to four microphones may be employed. In some platforms, the audio capture for a camcorder recording may also be performed using four microphones. In addition, many platforms are also using current-sense and voltage-sense feedback signals from the speaker for speaker protection. The current sense and voltage sense feedback signals may also be transported back in PDM format. Thus, it is quite common to have as many as eight digital microphones, or PDM signals received by a processor in a mobile device. For example, four digital microphones for noise-cancellation in voice communication, two digital microphones placed close to the camera for a camcorder, and two additional PDM signals for speaker protection feedback. In the future, it is possible that this need will increase.
Obviously, as the number of channels increases, the amount of die-area needed for storing the accumulator state for each PDM receiver grows linearly. For example, for a single microphone, 40 bytes of storage (5*8) may be needed, with the amount increasing to 320 bytes for eight microphones. In addition, if the a same compute engine (MAC) is used across all eight microphones (in order to save area), it requires 6*8*OSR or a 590 MHz clock for a 48 kHz sample rate. On the other hand, if the number of MAC units is increased, the area for the computational logic grows linearly.